1. Field of the Invention
The invention relates to a picture intermediate frequency circuit incorporated in a television receiver or the like.
2. Description of the Related Art
FIG. 4 is a circuit diagram of a conventional picture intermediate frequency circuit incorporated in a television receiver or the like. In the Figure, reference numeral 1 denotes an antenna; and reference numeral 2 denotes a tuner. The picture intermediate frequency signal output from terminal 2a of the tuner 2 is supplied to a picture signal processing circuit 8 via a matching circuit 3, an adjacent sound trap circuit 4, a pre-amplifier 5, a surface acoustic wave filter (hereinafter referred to as "SAW FILTER") 6, and a matching circuit 7.
Next, a simple description of the operation thereof will be given. A television signal or the like is input into the antenna 1. Then, at the tuner 2, a desired frequency of the television signal or the like is selected and amplified. A picture intermediate frequency signal produced by the conversion of the signal to that having a frequency range in the picture intermediate frequency region is output from the terminal 2a of the tuner 2.
The picture intermediate frequency signal output from the terminal 2a of the tuner 2 is input to the adjacent sound trap circuit 4 via the impedance matching circuit 3 for impedance matching. The trap circuit 4 is a typical notch filter circuit, which is called a bifilar T-type trap comprising a capacitor C4a and reactors L4a and L4b. It traps the channel sound carrier at the adjacent lower side of the picture signal to be received, and thereby removes interference in the picture to be received caused by the adjacent sound carrier.
The pre-amplifier 5 converts the impedance of the picture signal output from the adjacent sound trap circuit 4, and pre-amplifies the signal at a predetermined gain which precompensates for the attenuation to be caused by the SAW filter 6 (which follows the pre-amplifier 5). In the pre-amplifier 5, transistor Tr5a is a transistor comprising a buffer which converts impedance, and transistor Tr5b is a pre-amplifying transistor.
In the SAW filter 6, the predetermined frequency characteristic is given to the picture intermediate frequency signal, so that gain of the various transmitted signals can be adjusted by amount suitable for detecting vestigial side-band transmission signals. Then, the signal input to the picture signal processing circuit 8, via the impedance matching circuit 7 for impedance matching, is amplified to a picture intermediate frequency signal-this amplification method being well known. Then, the amplified signal is detected, afterwhich it is amplified and formed into to an AGC (Automatic Gain Control) voltage signal.
The picture signal SV output from the picture signal processing circuit 8 is introduced into terminal 9. The AGC voltage obtained at terminal 8a of the picture signal processing circuit 8 is supplied to AGC 2b of the tuner 2 via an AGC feedback circuit 10. Accordingly, a high-frequency amplifier of the tuner 2 is controlled such that it amplifies in accordance with the wave strength, which results in controlling the picture signal SV at a constant level irrespective of the wave strength.
Other conventional picture intermediate frequency circuits of television receivers and the like related to the present invention are disclosed, for example, in Japanese Patent Laid-Open No. 55-141871, Japanese Patent Laid-Open No. 52-71929, and Japanese Utility Model Laid-Open No. 61-162186.
Since the conventional picture intermediate frequency circuits have the above-described arrangement, the gain distribution of each of the amplifying components, namely, the tuner 2, the pre-amplifier 5, and the picture signal processing circuit 8 had to be set precisely. Otherwise, the picture intermediate frequency signal, particularly the signal at the pre-amplifier 5 in a strong electrical field, will exceed the dynamic range. This causes a high frequency signal produced in the non-linear region of the pre-amplifier 5 to interfere with a high frequency wave of the unnecessary adjacent picture signal present in the desired picture intermediate frequency of .+-.6 MHz (for example, in case of the NTSC system), which results in intermodulation, wherein a beat is mixed in the picture signal, so that it is difficult to see the picture clearly.